1. Field of the Invention
This invention relates to a sheet material conveying device adapted for use in a recording apparatus such as reproduction apparatus, laser beam printer, etc. More particularly, the invention is concerned with a conveying device capable of so controlling as to reduce jamming frequency of cut sheets as recording material in a conveying path of the recording apparatus when the sheets are fed irregularly through the conveying path such as a plurality of sheets being fed in superposition (hereinafter referred to as "plural feeding").
2. Description of Prior Arts
In these several years, printing speed of the reproduction apparatus and laser beam printer have been increased more and more, some of which are able to print as many as 100 sheets or more per minute of A-4 size paper. While such highspeed reproduction apparatus produces a large number of reproduced sheets per unit time, as an output, conveyance of the sheets for reproduction becomes unstable with the result that jamming frequency of the sheet tends to increase only for trifling reasons of its curling, moistening, and others. On account of this, it becomes difficult to make the best use of the high speed operation of the reproduction apparatus, and a throughput (or processing capability) of the machine is rather low. In addition, since the high speed machine receives a larger inertial load in its driving system than a low speed machine, a longer time is taken until the machine stops when the sheet jamming is detected. As the consequence of this, the feeding sheet is apt to be drawn into the conveying path in a longer length. On the other hand, if the jam detection timing is advanced, the machine indicates a jam detection with even a slight delay in the sheet feeding, and the machine is inevitably stopped in its operation in spite of the sheet having actually not been jammed.
Moreover, in the reproduction apparatus of a type, wherein cut sheets are used for the reproduction, plural feeding or slant feeding of the sheets causes jamming. In particular, plural feeding readily takes place when the surface of the cut sheet has a frictional coefficient different from the other surface thereof, i.e., it occurs on the surface of the cut sheet which has been in contact with wrapping paper for it. For instance, when a package of cut sheets consisting of 500 sheets is stacked in a large capacity sheet feeding device used in a high speed reproduction apparatus, such plural feeding of the sheets will take place at a rate of once every 500 sheets or so. Also, the plural feeding is prone to occur in the sheets stacked at the bottom portion of a paper feeding deck or paper feeding cassette, which readily brings about the jamming.
Usually, when plural feeding occurs, the numbers of sheet fed range from two to ten or so. As the number of sheets increases in the plural feeding, the sheet conveyance becomes difficult. In particular, when the feeding sheet is stopped for an instant as at a registering section in the reproduction apparatus, such instantaneous stoppage rather causes jamming. Even if there takes place no jamming at all, as the sheet cannot be uniformly sent out of the registering section, the sheet feeding is delayed, and this delay is determined as jam due to delay (or delay jam).